1. Field of the Invention
The present invention relates to an ink jet printer of a structure in which recording paper is transported through rotation of a feed roller while the paper is pressed against the feed roller by a leaf plate.
2. Description of the Related Art
As conventional ink jet printers widely in use, there are those of a type as shown in FIG. 10, in which, while a carrier 2 with an ink jet head 3 mounted thereon is moved in the direction transverse to printing paper P, which is transported by a paper feed mechanism 5 along a paper guide surface 8, ink is jetted from a nose portion 4 of the ink jet head 3 and printing is thereby performed. More specifically, an ink jet head 3 of a serial type is used and a desired image is formed on the printing paper P through main scanning performed by having such an ink jet head 3 moved by a head slide mechanism, not shown, and sub-scanning performed by having the printing paper P transported by the paper feed mechanism 5. In the example shown in FIG. 10, the paper feed mechanism 5 is constructed of a feed roller 6 rotationally driven by a motor, not shown, and a leaf plate 7 pressing the printing paper P against the feed roller 6.
In more concrete terms, the feed roller 6 is divided into three pieces in the direction transverse to the printing paper P so that its surface of friction with the printing paper P is limited to a minimum that is necessary and, thereby, the load on the motor is reduced. Further, the leaf plate 7 is structured so as to press the printing paper P sent over to the feed roller 6 through automatic or manual paper feeding against the feed roller 6 by its spring force. The leaf plate 7 is shaped, at the place where it faces the paper guide surface 8, to push the transported printing paper P to the paper guide surface 8, in order to secure good flatness of the printing paper P.
FIG. 11 shows a paper guide unit 21 on which the paper guide surface 8 is formed. In the paper guide unit 21, there are formed hollow portions 22 permitting the feed roller 6 divided into three pieces to be positioned therein. The hollow portion 22 extends into the region of the paper guide surface 8.
In the printer of the above described structure, the printing paper P transported to the paper guide surface 8 pressed against the feed roller 6 by the leaf plate 7 is printed with ink jetted from the nose portion 4 of the ink jet head 3, which is reciprocated in the direction transverse to the paper, and then, it is transported along the paper guide surface 8 and discharged into a paper stacker, not shown, on the downstream side.
Problems arising with the above mentioned prior art will be described below. In the ink jet printer, printing is performed with the nose portion of the ink jet head 3 in non-contacting relationship with the printing paper P and, further, very high printing density is required from it. Therefore, in order to improve the printing quality, the distance between the ink jet head 3 and the printing paper P must be maintained constant. However, in the ink jet printer mentioned above, because of the structure of the leaf plate 7 pressing the printing paper P against the feed roller 6, a pressure is applied to the paper and, thereby, undulations, in the form of continuous wave in the direction transverse to the paper, are produced. As a result, a problem arises that the distance between the printing paper P and the ink jet head 3 cannot be maintained constant and, hence, the printing quality is deteriorated. Further, with the apparatus in which the feed roller 6 is divided into a plurality of pieces as illustrated in FIG. 10 and FIG. 11, there are produced portions in the printing paper P to which the transporting force is applied and not applied, and also from such nonuniform application of the transporting force to the printing paper P, the problem of undulations occurring in the printing paper P leading to deterioration in the printing quality arises. Such undulations are also produced by moisture due to attachment of ink to the printing paper P while printing is performed with the ink jet head 3, i.e., especially when high-density printing is performed, there are produced differences in elongation and contraction of the paper on the printing side and the reverse side, and from this, undulations occur in the printing paper P. The undulations produced by the moisture of the ink spread to the surroundings and even reach the printing position. Also from the undulations thus produced, the problem of deterioration in the printing quality arises.
An example of undulations occurring from the above described causes is illustrated in FIG. 12. By the existence of the amplitude of such undulations, the nose portion 4 approaches or separates from the surface of the printing paper P, deviating from a regular distance between the surface of the printing paper P and the nose portion 4 and, thereby, deterioration in the printing quality is produced. The deterioration in the printing quality becomes very noticeable when ruled lines, for example, are printed. Sometimes, even an ink smear is produced on the printing paper P when a ridge portion of the undulation approaches very close to the nose portion 4 of the ink jet head 3. Specifically, as to the undulations produced in the printing paper P by moisture of the ink, the more severe the amplitude becomes, the higher the printing density on the printing paper P becomes. Accordingly, deterioration in the printing quality and ink smears occur frequently.
Applicant has tried to obtain good flatness of the printing paper by increasing the pressure of the leaf plate 7 or more precisely finishing the portion of the paper guide surface 8 shown in FIG. 11, but this was virtually useless.